1. Field of the Invention
The present disclosure relates to a capacitive sensor and a manufacturing method thereof, and more specifically, a miniature capacitive sensor device and manufacturing method thereof.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Most current capacitive sensors have fine capacitive structures formed on wafer substrates using the microelectromechanical systems (MEMS) technique, and have wide applications to many technical fields such as microphones, pressure gauges, accelerometer, oscillators, and RF switches.
The acoustic transducer, produced by a MEMS capacitive microphone chip which are integrated through silicon micro-machining technique and semiconductor processing technique, has the advantages of low weight, small volume, and good signal quality. As the request for sound quality of consumer electronic products such as telephone handsets has expanded increasingly and the markets for hearing aids have started to flourish as well, MEMS capacitive microphone chip has gradually become an important type of microphone chip.
Presently, the application of MEMS microphone chip is limited to a few types of MEMS microphone, because very few manufacturers currently produce MEMS microphones, including Knowles Corp., Infineon Corp., and Sonion Corp. And most of the MEMS microphone package processes for mass production are developed by Knowles Corp.
A MEMS microphone structure designed by Knowles Corp. is shown in FIGS. 1 to 3. An acoustic transducer 10 includes a conductive membrane 12 and a perforated member 40, both of which are supported by a base 30 and separated by an air gap 20. An air gap 22, extremely thin, is present between the conductive membrane 12 and the base 30, to enable the membrane 12 to move up and down freely and decouple the membrane 12 from the base 30. A plurality of indentations 13 are formed beneath the membrane 12, for avoiding stiction between the membrane 12 and the base 30.
Support portion 41 may be constructed of a ring or of a number of bumps. If the support portion 41 is constructed of a ring, a sound-sealed space is formed when the membrane 12 rests against the support portion 41, and as a result, the acoustic transducer exhibits a well-controlled low frequency roll-off. A dielectric layer 31 is provided between the air gap 22 and the base 30. A conducting electrode 42 is fixed beneath the nonconductive member 40. The member 40 has several holes 21 for creating a passageway 14 for sound flow.
A sacrificial layer as an interposer is interposed between the conductive membrane and the conducting electrode of the acoustic transducer (capacitive microphone device) designed by Knowles Corporation. Thereafter, the sacrificial layer is removed to form an air gap therein by an etching process. However, stiction easily occurs between the conductive membrane and the conducting electrode and causes them to short after the sacrificial layer is removed by etching. Because the conductive membrane is formed on the sacrificial layer and then the sacrificial layer is removed, residual stresses accumulate in the conductive membrane. Therefore, the sensitivity of the conductive membrane is reduced.
Even though such an acoustic transducer can use a conductive membrane with a special design to release the residual stresses therein and increase the sensitivity of the conductive membrane, the stiction problems still occur in the manufacturing processes. To conquer such problems, the acoustic transducer can utilize specially designed springs to balance and counteract the stiction force through the rigidity of the springs. In FIG. 4, another microphone structure designed by Knowles Corp. is shown. This structure is essentially the same as that of FIGS. 1 to 3, except the membrane 12 is connected to the base 30 via several spring structures 11 in order to decrease the intrinsic stress of the diaphragm and the stress generated from the base 30 or from the packaged device. The stresses existing in the membrane can be reduced, but the stiction problem still occurs therein. Accordingly, the manufacturing yield is low.
In addition to acoustic transducers, other capacitive sensors including pressure gauges, accelerators, oscillators, and RF switches also require small capacitance clearance to satisfy the requirements of a low driving voltage and high sensitivity. In the MEMS processes, the distance between two capacitance electrodes is based on a deposition step and the thickness of the sacrificial layer. However, the distance between the two electrodes is very small, approximately 1 to 3 μm, so the stiction problem frequently occurs therein. Accordingly, the manufacturing yield is very low.
In view of the above problems, there is an urgent need for improvements in the manufacture of capacitive sensors that can resolve the aforesaid problems of stiction and sensitivity.